Phase 4
is characterized by verified human-to-human transmission of an animal or
human-animal influenza reassortant virus able to cause “community-level
outbreaks.” The ability to cause sustained disease outbreaks in a community
marks a significant upwards shift in the risk for a pandemic. Any country
that suspects or has verified such an event should urgently consult with WHO
so that the situation can be jointly assessed and a decision made by the
affected country if implementation of a rapid pandemic containment operation
is warranted. Phase 4 indicates a significant increase in risk of a pandemic
but does not necessarily mean that a pandemic is a forgone conclusion.

Phase 5 is
characterized by human-to-human spread of the virus into at least two
countries in one WHO region. While most countries will not be affected at
this stage, the declaration of Phase 5 is a strong signal that a pandemic is
imminent and that the time to finalize the organization, communication, and
implementation of the planned mitigation measures is short.

Phase 6, the pandemic phase, is characterized by community level outbreaks
in at least one other country in a different WHO region in addition to the
criteria defined in Phase 5. Designation of this phase will indicate that a
global pandemic is under way.

During the
post-peak period,
pandemic disease levels in most countries with adequate surveillance will
have dropped below peak observed levels. The post-peak period signifies that
pandemic activity appears to be decreasing; however, it is uncertain if
additional waves will occur and countries will need to be prepared for a
second wave.

Previous
pandemics have been characterized by waves of activity spread over months.
Once the level of disease activity drops, a critical communications task
will be to balance this information with the possibility of another wave.
Pandemic waves can be separated by months and an immediate “at-ease” signal
may be premature.

In the
post-pandemic period,
influenza disease activity will have returned to levels normally seen for
seasonal influenza. It is expected that the pandemic virus will behave as a
seasonal influenza A virus. At this stage, it is important to maintain
surveillance and update pandemic preparedness and response plans
accordingly. An intensive phase of recovery and evaluation may be required.